bike with rear derailleur

Do you need the troublesome chain tensioner if you are mounting a motor to a 21speed bike with a rear derailleur? I was thinking of just mounting the engine in the perfect spot so the motors engine will have just the right amount of slack, and the rear derailleur will provide the bike's chain with the correct slack automatically?

Hi..
The only thing I will say about mounting a motor on a ""clustered gear" bike...is this. Through personal experience...after running you bike over hill an yon...the pawl in the rear hub begins to wear out. This IS a problem since the way we start our bikes (most of us I assume) is to pedal the bike. When that pawl wears out...it no longer catches the ratchet teeth on the interior of the hub...making it impossible to "pedal start" your bike, or facilitate the need to run like the wind and jump on. The bike I speak of was a 21 speed Mountain bike, putting 4800 miles on it and wore out 2 rims. At $65 a pop for a rim...tends to get pricey after a while. Now...I use only single speed beach cruisers. Easy to pedal...no rear hub worries.
Hope this bit of info helps.